IMHO there are two categories of mastering software. The host programs (e.g., Wavelab, Sound Forge, Samplitude, ) and mastering plugins (e.g., Waves Ltd, Ozone, Steinberg Mastering Edition, and a bunch of other ones).
Many of the host programs also provide some native plug-ins that can be used for Mastering.
Of course, price is always an issue. In my experience I have been getting pretty good results using Wavelab for the host program and the Waves Gold Bundle for my mastering plugins (L1 and C4 are great) - but it is an expensive combination.
Sound Forge together with Ozone would be a much cheaper way to go, if price is an issue.
Also, you could probably use your multitracking program as the host if you wanted, then you would only need some mastering plugins. Not my preferred approach (as generally you will have less tools available to you) but it can work and will save you some money.
So, how much did you want to spend?